Measuring the genetic influence in modulating the human life span: gene-environment interaction and the sex-specific genetic effect

Associations between STR autosomal markers and longevity

Life span is a complex and multifactorial trait, which is shaped by genetic, epigenetic, environmental, and stochastic factors. The possibility that highly hypervariable short tandem repeats (STRs) associated with longevity has been largely explored by comparing the genotypic pools of long lived and younger individuals, but results so far have been contradictory. In view of these contradictory findings, the present study aims to investigate whether HUMTHO1 and HUMCSF1PO STRs, previously associated with longevity, exert a role as a modulator of life expectancy, as well as to assess the extent to which other autosomal STR markers are associated with human longevity in population from northern Spain. To that end, 21 autosomal microsatellite markers have been studied in 304 nonagenarian individuals (more than 90 years old) and 516 younger controls of European descent. Our results do not confirm the association found in previous studies between longevity and THO1 and CSF1PO loci. However, significant association between longevity and autosomal STR markers D12S391, D22S1045, and DS441 was observed. Even more, when we compared allelic frequency distribution of the 21 STR markers between cases and controls, we found that 6 out of the 21 STRs studied showed different allelic frequencies, thus suggesting that the genomic portrait of the human longevity is far complex and probably shaped by a high number of genomic loci.

Age- and gender-specific epistasis between ADA and TNF-α influences human life-expectancy

Aging is a complex phenotype with multiple determinants but a strong genetic component significantly impacts on survival to extreme ages. The dysregulation of immune responses occurring with increasing age is believed to contribute to human morbidity and mortality. Conversely, some genetic determinants of successful aging might reside in those polymorphisms for the immune system genes regulating immune responses. Here we examined the main effects of single loci and multi-locus interactions to test the hypothesis that the adenosine deaminase (ADA) and tumor necrosis factor alpha (TNF-α) genes may influence human life-expectancy. ADA (22G>A, rs73598374) and TNF-α (-308G>A, rs1800629; -238G>A, rs361525) functional SNPs have been determined for 1071 unrelated healthy individuals from Central Italy (18-106 years old) divided into three gender-specific age classes defined according to demographic information and accounting for the different survivals between sexes: for men (women), the first class consists of individuals<66 years old (<73 years old), the second class of individuals 66-88 years old (73-91 years old), and the third class of individuals>88 years old (>91 years old). Single-locus analysis showed that only ADA 22G>A is significantly associated with human life-expectancy in males (comparison 1 (age class 2 vs. age class 1), O.R. 1.943, P=0.036; comparison 2 (age class 3 vs. age class 2), O.R. 0.320, P=0.0056). Age- and gender-specific patterns of epistasis between ADA and TNF-α were found using Generalized Multifactor Dimensionality Reduction (GMDR). In comparison 1, a significant two-loci interaction occurs in females between ADA 22G>A and TNF-α -238G>A (Sign Test P=0.011). In comparison 2, both two-loci and three-loci interaction are significant associated with increased life-expectancy over 88 years in males. In conclusion, we report that a combination of functional SNPs within ADA and TNF-α genes can influence life-expectancy in a gender-specific manner and that males and females follow different pathways to attain longevity.

Power assessment for genetic association study of human longevity using offspring of long-lived subjects

Recently, an indirect genetic association approach that compares genotype frequencies in offspring of long-lived subjects and offspring from random families has been introduced to study gene-longevity associations. Although the indirect genetic association has certain advantages over the direct association approach that compares genotype frequency between centenarians and young controls, the power has been of concern. This paper reports a power study performed on the indirect approach using computer simulation. We perform our simulation study by introducing the current Danish population life table and the proportional hazard model for generating individual lifespan. Family genotype data is generated using a genetic linkage program for given SNP allele frequency. Power is estimated by setting the type I error rate at 0.05 and by calculating the Armitage's chi-squared test statistic for 200 replicate samples for each setting of the specified allele risk and frequency parameters under different modes of inheritance and for different sample sizes. The indirect genetic association analysis is a valid approach for studying gene-longevity association, but the sample size requirement is about 3-4 time larger than the direct approach. It also has low power in detecting non-additive effect genes. Indirect genetic association using offspring from families with both parents as nonagenarians is nearly as powerful as using offspring from families with one centenarian parent. In conclusion, the indirect design can be a good choice for studying longevity in comparison with other alternatives, when relatively large sample size is available.

Gender-specific association of ADA genetic polymorphism with human longevity

Aim of this study was to investigate whether the polymorphic ADA (Adenosine Deaminase, EC 3.5.4.4) gene, which determines the cellular level of adenosine and plays a crucial role in the regulation of the immune system and in the control of metabolic rates, is involved in longevity. 884 unrelated healthy individuals (age range 10-106 years, 400 males and 484 females) from central Italy were studied. ADA genotyping was performed by RFLP-PCR. Frequency distributions were compared using the chi-square test and a three-way contingency table analysis by a log linear model was applied to test independence between the variables. We found that ADA influences human life-span in a sex and age specific way. An increased frequency of ADA*2 carriers was found in males aged 80-85, and a decreased frequency in males over 85 (chi(2) = 13.93; df = 3; P = 0.003); significant differences among the age groups was not found in females. A strong interaction among age groups, ADA genotype and sex (G = 15.086; df = 3; P = 0.0017) was found. Males aged 80-85 could be protected from ischemic stroke by higher levels of adenosine (determined by the ADA*2 allele). The decrease of ADA*2 carriers in males over 85 may depend essentially on immunological factors; reduced levels of adenosine protect from asthma and other pulmonary diseases and lead to a reduced activation of inflammatory cells and pro-inflammatory cytokines production. Moreover, the low level of adenosine may potentiate the activity of NK and other cellular effectors against tumor cells. The negligible effect of ADA genetic polymorphism in females suggest a marginal influence of genetic factors in determining longevity in this sex, confirming previous reports.

Power for genetic association study of human longevity using the case-control design

The efficiency of the popular case-control design in gene-longevity association studies needs to be verified because, different from a binary trait, longevity represents only the extreme end of the continuous life span distribution without a clear cutoff for defining the phenotype. In this paper, the authors use the current Danish life tables to simulate individual life span by using a variety of scenarios and assess the empirical power for different sample sizes when cases are defined as centenarians or as nonagenarians. Results show that, although using small samples of centenarians (several hundred) provides power to detect only common alleles with large effects (a >20% reduction in hazard rate), large samples of centenarians (>1,000) achieve power to capture genes responsible for minor effects (5%-10% hazard reduction depending on the mode of inheritance). Although the method provides good power for rare alleles with multiplicative or dominant effects, it performs poorly for rare recessive alleles. Power is drastically reduced when nonagenarians are considered cases, with a more than 5-fold difference in the size of the case sample required to achieve comparable power as that found with centenarians.

Buffering mechanisms in aging: a systems approach toward uncovering the genetic component of aging

An unrealized potential to understand the genetic basis of aging in humans, is to consider the immense survival advantage of the rare individuals who live 100 years or more. The Longevity Gene Study was initiated in 1998 at the Albert Einstein College of Medicine to investigate longevity genes in a selected population: the “oldest old” Ashkenazi Jews, 95 years of age and older, and their children. The study proved the principle that some of these subjects are endowed with longevity-promoting genotypes. Here we reason that some of the favorable genotypes act as mechanisms that buffer the deleterious effect of age-related disease genes. As a result, the frequency of deleterious genotypes may increase among individuals with extreme lifespan because their protective genotype allows disease-related genes to accumulate. Thus, studies of genotypic frequencies among different age groups can elucidate the genetic determinants and pathways responsible for longevity. Borrowing from evolutionary theory, we present arguments regarding the differential survival via buffering mechanisms and their target age-related disease genes in searching for aging and longevity genes. Using more than 1,200 subjects between the sixth and eleventh decades of life (at least 140 subjects in each group), we corroborate our hypotheses experimentally. We study 66 common allelic site polymorphism in 36 candidate genes on the basis of their phenotype. Among them we have identified a candidate-buffering mechanism and its candidate age-related disease gene target. Previously, the beneficial effect of an advantageous cholesteryl ester transfer protein (CETP-VV) genotype on lipoprotein particle size in association with decreased metabolic and cardiovascular diseases, as well as with better cognitive function, have been demonstrated. We report an additional advantageous effect of the CETP-VV (favorable) genotype in neutralizing the deleterious effects of the lipoprotein(a) (LPA) gene. Finally, using literature-based interaction discovery methods, we use the set of longevity genes, buffering genes, and their age-related target disease genes to construct the underlying subnetwork of interacting genes that is expected to be responsible for longevity. Genome wide, high-throughput hypothesis-free analyses are currently being utilized to elucidate unknown genetic pathways in many model organisms, linking observed phenotypes to their underlying genetic mechanisms. The longevity phenotype and its genetic mechanisms, such as our buffering hypothesis, are similar; thus, the experimental corroboration of our hypothesis provides a proof of concept for the utility of high-throughput methods for elucidating such mechanisms. It also provides a framework for developing strategies to prevent some age-related diseases by intervention at the appropriate level.

Previous research showed that the frequency of deleterious genotype of some age-related disease decreases its prevalence as the population ages, as expected, since subjects with deleterious genotype are weeded out due to mortality. There exists, however, a set of age-related genes whose deleterious genotype indeed decreases up to ages 80–85, but subsequently increases monotonically, until by age 100 its prevalence is similar to that at age ∼60. Why is a known harmful genotype so prevalent among centenarians? Most likely because this genotype is protected by longevity genes. We corroborated this hypothesis by studying gene–gene interactions between age-related disease genotypes and longevity genotypes. Our findings suggest that individuals with the favorable longevity genotype can have just as many deleterious aging genotypes as the rest of the population because their longevity genotype protects them from the harmful effects of the other. We identify genes contributing to extreme lifespan as well as their counterpart, age-related disease genes. Our findings provide a proof of concept for the utility of high-throughput methods, and for elucidating mechanisms by which longevity genes buffer the effects of disease genes. Our approach gives hope for developing new medications that will protect against several age-related diseases.

Theories of biological aging: genes, proteins, and free radicals

Traditional categorization of theories of aging into programmed and stochastic ones is outdated and obsolete. Biological aging is considered to occur mainly during the period of survival beyond the natural or essential lifespan (ELS) in Darwinian terms. Organisms survive to achieve ELS by virtue of genetically determined longevity assuring maintenance and repair systems (MRS). Aging at the molecular level is characterized by the progressive accumulation of molecular damage caused by environmental and metabolically generated free radicals, by spontaneous errors in biochemical reactions, and by nutritional components. Damages in the MRS and other pathways lead to age-related failure of MRS, molecular heterogeneity, cellular dysfunctioning, reduced stress tolerance, diseases and ultimate death. A unified theory of biological aging in terms of failure of homeodynamics comprising of MRS, and involving genes, milieu and chance, is acquiring a definitive shape and wider acceptance. Such a theory also establishes the basis for testing and developing effective means of intervention, prevention and modulation of aging.

Sex and Age Specificity of Susceptibility Genes Modulating Survival at Old Age

OBJECTIVE

We aimed to investigate the influence of the genetic variability of candidate genes on survival at old age in good health.

METHODS

First, on the basis of a synthetic survival curve constructed using historic mortality data taken from the Italian population from 1890 onward, we defined three age classes ranging from 18 to 106 years. Second, we assembled a multinomial logistic regression model to evaluate the effect of dichotomous variables (genotypes) on the probability to be assigned to a specific category (age class). Third, we applied the regression model to a cross-sectional dataset (10 genes; 972 subjects selected for healthy status) categorized according to age and sex.

RESULTS

We found that genetic factors influence survival at advanced age in good health in a sex- and age-specific way. Furthermore, we found that genetic variability plays a stronger role in males than in females and that, in both genders, its impact is especially important at very old ages.

CONCLUSIONS

The analyses presented here underline the age-specific effect of the gene network in modulating survival at advanced age in good health.

Design and analysis in genetic studies of human ageing and longevity

With the success of the Human Genome Project and taking advantage of the recent developments in high-throughput genotyping techniques as well as in functional genomics, it is now feasible to collect vast quantities of genetic data with the aim of deciphering the genetics of human complex traits. As a result, the amount of research on human ageing and longevity has been growing rapidly in recent years. The situation raises questions concerning efficient choice of study population, sampling schemes, and methods of data analysis. In this article, we summarize the key issues in genetic studies of human ageing and longevity ranging from research design to statistical analyses. We discuss the virtues and drawbacks of the multidisciplinary approaches including the population-based cross-sectional and cohort studies, family-based linkage analysis, and functional genomics studies. Different analytical approaches are illustrated with their performances compared. In addition, important research topics are highlighted together with experiment design and data analyzing issues to serve as references for future studies.

Genes and longevity: a genetic-demographic approach reveals sex- and age-specific gene effects not shown by the case–control approach (APOE and HSP70.1 loci)

Association analyses between gene variability and human longevity carried out by comparing gene frequencies between population samples of different ages (case/control design) may provide information on genes and pathways playing a role in modulating survival at old ages. However, by dealing with cross-sectional data, the gene-frequency (GF) approach ignores cohort effects in population mortality changes. The genetic-demographic (GD) approach adds demographic information to genetic data and allows the estimation of hazard rates and survival functions for candidate alleles and genotypes. Thus mortality changes in the cohort to which the cross-sectional sample belongs are taken into account. In this work, we applied the GD method to a dataset relevant to two genes, APOE and HSP70.1, previously shown to be related to longevity by the GF method. We show that the GD method reveals sex- and age-specific allelic effects not shown by the GF analysis. In addition, we provide an algorithm for the implementation of a non-parametric GD analysis.

Mitochondrial DNA involvement in human longevity

The main message of this review can be summarized as follows: aging and longevity, as complex traits having a significant genetic component, likely depend on a number of nuclear gene variants interacting with mtDNA variability both inherited and somatic. We reviewed the data available in the literature with particular attention to human longevity, and argued that what we hypothesize for aging and longevity could have a more general relevance and be extended to other age-related complex traits such as Alzheimer's and Parkinson's diseases. The genetics which emerges for complex traits, including aging and longevity, is thus even more complicated than previously thought, as epistatic interactions between nuclear gene polymorphisms and mtDNA variability (both somatic and inherited) as well as between mtDNA somatic mutations (tissue specific) and mtDNA inherited variants (haplogroups and sub-haplogroups) must be considered as additional players capable of explaining a part of the aging and longevity phenotype. To test this hypothesis is one of the main challenge in the genetics of aging and longevity in the next future.

Heat-Shock Protein 70 Genes and Human Longevity: A View from Denmark

We have studied the association of three single nucleotide polymorphisms (SNPs) present in the three HSP70 (heat-shock protein) genes on 6p21 with human longevity. The availability of biological samples from various population cohorts in Denmark has given us the opportunity to try novel methods of gene association with human longevity. A significant association of one haplotype with male longevity was observed. Furthermore, a significant difference in the survival of the carriers of the different genotypes in females was observed. We also found an age-dependant decline in the ability of peripheral blood mononuclear cells to respond to heat stress in terms of Hsp70 induction.

Genetic association analysis of human longevity in cohort studies of elderly subjects: an example of the PON1 gene in the Danish 1905 birth cohort

Although the case-control or the cross-sectional design has been popular in genetic association studies of human longevity, such a design is prone to false positive results due to sampling bias and a potential secular trend in gene-environment interactions. To avoid these problems, the cohort or follow-up study design has been recommended. With the observed individual survival information, the Cox regression model has been used for single-locus data analysis. In this article, we present a novel survival analysis model that combines population survival with individual genotype and phenotype information in assessing the genetic association with human longevity in cohort studies. By monitoring the changes in the observed genotype frequencies over the follow-up period in a birth cohort, we are able to assess the effects of the genotypes and/or haplotypes on individual survival. With the estimated parameters, genotype- and/or haplotype-specific survival and hazard functions can be calculated without any parametric assumption on the survival distribution. In addition, our model estimates haplotype frequencies in a birth cohort over the follow-up time, which is not observable in the multilocus genotype data. A computer simulation study was conducted to specifically assess the performance and power of our haplotype-based approach for given risk and frequency parameters under different sample sizes. Application of our method to paraoxonase 1 genotype data detected a haplotype that significantly reduces carriers' hazard of death and thus reveals and stresses the important role of genetic variation in maintaining human survival at advanced ages.

Understanding and modulating ageing

Ageing is characterized by a progressive accumulation of molecular damage in nucleic acids, proteins and lipids. The inefficiency and failure of maintenance, repair and turnover pathways is the main cause of age-related accumulation of damage. Research in molecular gerontology is aimed at understanding the genetic and epigenetic regulation of survival and maintenance mechanisms at the levels of transcription, post-transcriptional processing, post-translational modifications, and interactions among various gene products. Concurrently, several approaches are being tried and tested to modulate ageing in a wide variety of organisms. The ultimate aim of such studies is to improve the quality of human life in old age and prolong the health-span. Various gerontomodulatory approaches include gene therapy, hormonal supplementation, nutritional modulation and intervention by free radical scavengers and other molecules. A recent approach is that of applying hormesis in ageing research and therapy, which is based on the principle of stimulation of maintenance and repair pathways by repeated exposure to mild stress. A combination of molecular, physiological and psychological modulatory approaches can realize "healthy ageing" as an achievable goal in the not-so-distant future.

Toward generation XL: anthropometrics of longevity in late 20th-century United States

All-cause and cause-specific mortality among white U.S. men and women are analyzed using the NHANES I data (1971-1975) and epidemiologic follow-up to 1992, to examine the effect of physical stature on mortality, controlling for other confounding variables within a discrete-time framework. We find an association between mortality and both body mass index (BMI) and height, but the height effect is sensitive with respect to the age range under consideration. Although the resulting minimum-mortality BMI is higher than the widely accepted healthy range, the recent increase in weight implies that further gains in life expectancy are unlikely to derive from the anthropometry-mortality relationship.

Estimating haplotype relative risks on human survival in population-based association studies

Association-based linkage disequilibrium (LD) mapping is an increasingly important tool for localizing genes that show potential influence on human aging and longevity. As haplotypes contain more LD information than single markers, a haplotype-based LD approach can have increased power in detecting associations as well as increased robustness in statistical testing. In this paper, we develop a new statistical model to estimate haplotype relative risks (HRRs) on human survival using unphased multilocus genotype data from unrelated individuals in cross-sectional studies. Based on the proportional hazard assumption, the model can estimate haplotype risk and frequency parameters, incorporate observed covariates, assess interactions between haplotypes and the covariates, and investigate the modes of gene function. By introducing population survival information available from population statistics, we are able to develop a procedure that carries out the parameter estimation using a nonparametric baseline hazard function and estimates sex-specific HRRs to infer gene-sex interaction. We also evaluate the haplotype effects on human survival while taking into account individual heterogeneity in the unobserved genetic and nongenetic factors or frailty by introducing the gamma-distributed frailty into the survival function. After model validation by computer simulation, we apply our method to an empirical data set to measure haplotype effects on human survival and to estimate haplotype frequencies at birth and over the observed ages. Results from both simulation and model application indicate that our survival analysis model is an efficient method for inferring haplotype effects on human survival in population-based association studies.

A novel VNTR enhancer within the SIRT3 gene, a human homologue of SIR2, is associated with survival at oldest ages

SIR2 genes control life span in model organisms, playing a central role in evolutionarily conserved pathways of aging and longevity. We wanted to verify whether similar effects may act in humans too. First, we searched for variability in the human sirtuin 3 gene (SIRT3) and discovered a VNTR polymorphism (72-bp repeat core) in intron 5. The alleles differed both for the number of repeats and for presence/absence of potential regulatory sites. Second, by transient transfection experiments, we demonstrated that the VNTR region has an allele-specific enhancer activity. Third, by analyzing allele frequencies as a function of age in a sample of 945 individuals (20-106 years), we found that the allele completely lacking enhancer activity is virtually absent in males older than 90 years. Thus the underexpression of a human sirtuin gene seems to be detrimental for longevity as it occurs in model organisms.

The empirical relationship between longevity and physical stature may be obscured by unobserved genetic diversity in the optimal metabolic rate

It is hypothesized that an inheritable "optimal" metabolic rate is associated with the lowest attainable mortality risk, greatest attainable height (in youth) and a desirable body weight. Positive deviations from this rate are reflected in overweight and higher mortality, negative deviations in shorter stature (stunting), low body mass index (BMI), and higher mortality. In a heterogeneous population the optimal rate may differ among genotypes, and those with a higher optimal rate are the ones who can translate more energy intake into growth in height such that under optimal net-nutritional intake, they are able to reach a greater final adult height with a lower body mass index than genotypes with a lower optimal rate. Hence, the optimal height and weight - in terms of longevity - may differ substantially among genotypes, so that an empirical estimation of the relationship may be blurred in a heterogeneous society. Conversely, the "true" association (within genotypes) may be stronger than the measured one, which may have some implications for the contribution of the secular increase in human height to the rise in life expectancy or the optimal BMI in persons who experienced stunting in youth.

Logistic regression models for polymorphic and antagonistic pleiotropic gene action on human aging and longevity

In this paper, we apply logistic regression models to measure genetic association with human survival for highly polymorphic and pleiotropic genes. By modelling genotype frequency as a function of age, we introduce a logistic regression model with polytomous responses to handle the polymorphic situation. Genotype and allele-based parameterization can be used to investigate the modes of gene action and to reduce the number of parameters, so that the power is increased while the amount of multiple testing minimized. A binomial logistic regression model with fractional polynomials is used to capture the age-dependent or antagonistic pleiotropic effects. The models are applied to HFE genotype data to assess the effects on human longevity by different alleles and to detect if an age-dependent effect exists. Application has shown that these methods can serve as useful tools in searching for important gene variations that contribute to human aging and longevity.

Power of non-parametric linkage analysis in mapping genes contributing to human longevity in long-lived sib-pairs

This report investigates the power issue in applying the non-parametric linkage analysis of affected sib-pairs (ASP) [Kruglyak and Lander, 1995: Am J Hum Genet 57:439-454] to localize genes that contribute to human longevity using long-lived sib-pairs. Data were simulated by introducing a recently developed statistical model for measuring marker-longevity associations [Yashin et al., 1999: Am J Hum Genet 65:1178-1193], enabling direct power comparison between linkage and association approaches. The non-parametric linkage (NPL) scores estimated in the region harboring the causal allele are evaluated to assess the statistical power for different genetic (allele frequency and risk) and heterogeneity parameters under various sampling schemes (age-cut and sample size). Based on the genotype-specific survival function, we derived a heritability calculation as an overall measurement for the effect of causal genes with different parameter settings so that the power can be compared for different modes (dominant, recessive) of inheritance. Our results show that the ASP approach is a powerful tool in mapping very strong effect genes, both dominant and recessive. To map a rare dominant genetic variation that reduces hazard of death by half, a large sample (above 600 pairs) with at least one extremely long-lived (over age 99) sib in each pair is needed. Again, with large sample size and high age cut-off, the method is able to localize recessive genes with relatively small effects, but the power is very limited in case of a dominant effect. Although the power issue may depend heavily on the true genetic nature in maintaining survival, our study suggests that results from small-scale sib-pair investigations should be referred with caution, given the complexity of human longevity.

A mechanistic model for the aging of human skin

The microinflammatory model of skin aging is described. This model accounts for the loss of elasticity, resiliene and flexibility of the dermis, as well as for the appearance of wrinkles and for the thinning of the epidermis which are associated with aging. The lack of appropriate apparatuses for the correct measurements of skin hydration does not allow one to test this model for its predictive capability of the appearance of dry skin with age. The micro-inflammatory model of skin aging fails to predict the appearance of age spots on the surface of the skin.

The study of APOA1, APOC3 and APOA4 variability in healthy ageing people reveals another paradox in the oldest old subjects

The genes coding for apolipoprotein A1 (APOA1), apolipoprotein C3 (APOC3) and apolipoprotein A4 (APOA4) are tandemly organised within a short region on chromosome 11q23-q24. Polymorphisms of these genes have been extensively investigated in lipoprotein disorders and cardiovascular diseases, but poorly investigated in healthy ageing. The aim of this study was to describe possible modifications of the APOA1, APOC3, and APOA4 gene pool by cross-sectional studies carried out in a healthy ageing population whose ages ranged from 18 to 109 years (800 subjects, 327 males and 473 females, free of clinically manifested disease, and with emato-chemical parameters in the norm). APOA1-MspI-RFLP (-75 nt from the transcription starting site), APOC3-SstI-RFLP (3'UTR, 3238 nt), and APOA4-HincII-RFLP (Asp127/Ser127) were analysed according to age and sex. A significant age-related variation of the APOA1 gene pool was observed in males. An analysis of the allele average effect exerted by APOA1-MspI-RFLP A/P alleles (Absence/Presence of the restriction site) on lipidemic parameters in 46-80 year old males showed that allele A decreased, while allele P significantly increased, serum LDL-cholesterol. Unexpectedly, the P allele was over-represented in the group of the oldest old subjects, thus giving evidence of another "genetic paradox of centenarians".

The influences on human longevity by HUMTHO1.STR polymorphism (Tyrosine Hydroxylase gene)

A new method based on the recently developed relative risk approach is introduced, and applied to data from Italian centenarian study (965 subjects aged from 13 to 109 years old) for investigating influences on longevity by Tyrosine Hydroxylase (TH) gene variability. The strategic parameterization enables the model to disentangle the various ways by which HUMTHO1.STR alleles (alleles 6, 7, 8, 9, 10*, 10, as defined according to the number of repeats) may contribute in reducing or increasing the hazard of death with different patterns of influences. Among all the alleles, we have found that allele 10* (10 imperfect repeats) shows a remarkable dominant and beneficial effect that reduces the log hazard of death in an additive manner. The results confirm that HUMTHO1.STR polymorphism is involved in the modulation of human longevity.

Population genomics: ageing by association

Mutations in the gene Klotho lead to premature ageing in mice. Recent work on human genetic variation has identified an association between a particular allele of Klotho and human lifespan. A harbinger of things to come, this work illustrates the power and growth potential for association studies of human ageing.

A case-only approach for assessing gene by sex interaction in human longevity

As one aspect of the complex feature of longevity, gene by sex interaction plays an important role in influencing human life span. With advances in molecular genetics, more studies aimed at assessing gene by sex interaction are expected. New and valid statistical methods are needed. In this article, we introduce a nontraditional approach, the case-only design, which was originally proposed for assessing gene and disease associations, to detect gene by sex interaction in human longevity. Applications of this method to data collected from centenarian studies show that it can produce consistent results as compared with results obtained from case-control and other approaches. The method cannot be used as a substitute for traditional case-control studies since it is limited to the detection of interactions only. However, the easily applicable case-only approach can be an important tool for screening many potential genes that contribute to human longevity.

A centenarian-only approach for assessing gene-gene interaction in human longevity

In this study, we introduce a centenarian-only approach to the assessment of gene-gene interaction that contributes to human longevity. This approach corresponds to the non-traditional case-only method in the genetic study of gene and disease associations. We first describe how the method can be implemented to screen for gene-gene interaction in human longevity. Then we apply the method to centenarian data collected from an Italian centenarian study in order to detect the interactions between the REN gene and the mitochondrial haplotypes. A significant interaction between REN gene allele 10 and the mitochondrial H haplotype, which may favour longevity, was found. Important features of the application in human longevity studies are highlighted and discussed. Since centenarians constitute a special population representing successful ageing, the centenarian-only approach will be an important tool in the search for major genes that contribute to human longevity.

A logistic regression model for measuring gene-longevity associations

The logistic regression model is a popular model for data analysis in epidemiological research. In this paper, we use this model to analyze genetic data collected from gene-longevity association studies. This new approach models the probability of observing one genotype as a function of the age of investigated individuals. Applying the model to genotype data on the TH and 3'ApoB-VNTR loci collected from an Italian centenarian study, we show how it can be used to model the different ways that genes affect survival, including sex- and age-specific influences. We highlight the advantages of this application over other available models. The application of the model to empirical data indicates that it is an efficient and easily applicable approach for determining the influences of genes on human longevity.

Sex-specific longevity associations defined by Tyrosine Hydroxylase-Insulin-Insulin Growth Factor 2 haplotypes on the 11p15.5 chromosomal region

By studies in centenarians, it was recently found that an STR marker of the Tyrosine Hydroxylase (TH, 11p15.5) gene is associated with human longevity. The aim of the present study was to continue the exploration of the 11p15.5 chromosomal region in human longevity by analyzing two additional RFLP markers, which lie in the Insulin (INS) and Insulin Growth Factor 2 (IGF2) genes. Both the genes, which are localized downstream TH, are indeed good candidates in longevity, as ascertained on the basis of laboratory studies in experimental models. Neither INS nor IGF2 markers did reveal association with longevity. Nevertheless, linkage disequilibrium analyses showed sex-specific longevity associations defined by both TH-INS and TH-IGF2 haplotypes. On the whole, the results reinforce the involvement of the chromosomal region spanning from TH to IGF2 loci in controlling the longevity phenotype in humans.

Variations of cardiovascular disease associated genes exhibit sex-dependent influence on human longevity

This article investigates the relationship between the polymorphic variations in genes associated with cardiovascular disease and longevity in the Danish population. A new procedure that combines both demographic and the individual genetic information in determining the relative risks of the observed genetic variations is applied. The sex-dependent influences can be found by introducing sex-specific population survival and incorporating the risk of gene-sex interaction. Three genetic polymorphisms, angiotensinogen M/T235, blood coagulation factor VII (FVII) R/Q353 and FVII-323ins10, manifest significant influences on survival in males, with reduced hazards of death for carriers of the angiotensinogen M235 allele, the F VII Q353 allele, and the FVII-323P10 allele. The results show that some of these genotypes associated with lower risk of CVD could also reduce the carrier's death rate and contribute to longevity. However, the presence of sex-dependent effects and the fact that major CVD-associated genes failed to impose detrimental influence on longevity lead us to concur that the aging process is highly complicated.